1. Field of the Invention
The present invention relates to an improvement in a semiconductor laser. The improvement is for obtaining lasers with a low threshold current and stable transverse mode lasing.
2. Description of the Prior Art
Semiconductor lasers have many advantages such as smallness, high efficiency, direct modulation by its driving current, and etc., and therefore have bright future as light sources for optical communication and optical data processing. Laser for such use necessitates characteristics of stable fundamental transverse mode lasing, low threshold current, high output of light and high reliability.
A representative example of the conventional semiconductor laser of InGaAsP/InP double heterostructure planar type is shown in FIG. 1, which is a sectional elevation view by a plane perpendicular to output light thereof. The conventional laser of FIG. 1 is made by sequential liquid phase epitaxial growth wherein on
a substrate 1 of: n-InP, PA1 a first clad layer 2 of: n-InP, PA1 an active layer 3 of: In.sub.0.28 Ga.sub.0.72 As.sub.0.62 P.sub.0.38, PA1 a second clad layer 4 of: p-InP, and PA1 a cap layer 5 of: n-In.sub.0.28 Ga.sub.0.72 As.sub.0.62 P.sub.0.38. PA1 ohmic contact region 6 of p.sup.+ -type
the following layers are sequentially formed:
Then, on the cap layer 5, a Zn-diffused stripe-shaped
is formed.
Thereafter, Au-Zn ohmic electrode 7 and Au-Ge-Ni ohmic electrode 8 are formed.
The abovementioned conventional semiconductor laser of FIG. 1 does not comprise a particular distribution of refractive index in the direction parallel to the junction face, which is between the layers 4 and 5, and therefore, as the injected current increases its transverse mode becomes instable, thereby inducing practically undesirable phenomena of higher mode oscillation or kink in the current--light output characteristic curve. Besides, threshold current of the abovementioned conventional laser is large since the injected current is liable to spread transversely.